A HIGH EFFICIENCY RECEIVER 



141 



as a tube of this character, viz., -^ c.g.s. units, where p is the density 

 of air, c, the velocity of sound, and A, the area.- 



The Coupling Air Chamber and Diaphragm 

 In order efifectively to make use of horns as sound intensifiers it is 

 usually necessary to couple the throat of the horn to the diaphragm 

 through an air chamber. We shall first consider the effect of this air 

 chamber on the sound output of the loud speaker. This coupling air 

 chamber is generally of an indefinite conical shape of the type shown 

 in Fig. 1. If we assume that this air chamber is so proportioned that 



■THROAT OF HORN 



DIAPHRAGM 



IL_^ L 



Conventional type of coupling air chamber. 



the annular area, lirrt, is equal to the throat area, and that the di- 

 aphragm is driven so that its displacement is paraboloidal, then, as 

 calculated from formulae developed in appendix A, the mechanical im- 

 pedance imposed by the air chamber and horn on the diaphragm is 

 shown in the curves of Fig. 2. Here the ordinates of the curves ri 

 and Xi are proportional to the resistance and reactance respectively, 

 and the abscissae are equal to the product of the frequency and the 

 radius of the diaphragm. Of particular interest here is the large 

 decrease in the resistance with frequency, for ri, multiplied by the 

 square of the velocity of the diaphragm, is the acoustic power delivered 

 to the horn. For example, if the radius of the diaphragm were four 

 centimeters, no sound would be emitted at 4,000 p.p.s. We have here 

 one reason why most horn-type loud speakers fail to reproduce high 

 frequency tones at sufficient intensity. Of course, in most cases the 

 high frequency tones are further attenuated by the fact that the mode 

 of motion of the diaphragm changes with frequency. The decrease in 



- "The Function and Design of Horns," by C. R. Hanna and J. Slepian, Journal 

 of the A. I. E. E., March 1924. 



